Wireless communication devices have become smaller and more powerful in order to meet consumer needs and to improve portability and convenience. Consumers have become dependent upon wireless communication devices such as cellular telephones, personal digital assistants (PDAs), laptop computers, and the like. Consumers have come to expect reliable service, expanded areas of coverage, and increased functionality.
A common way to conserve power in a wireless communication device is by placing the electronic device in a low power consumption state (e.g., “sleep mode”) when the wireless communication device is not in use. When a wireless communication device is in a sleep mode, a sleep clock may be used to track time and schedule when the wireless communication device “wakes up.”
Clocks traditionally include a resonator-based oscillator, such as a low frequency crystal oscillator (e.g., the 32 kilohertz (kHz) oscillator commonly found in wristwatches). However, the size and cost of crystal oscillators make them ill-suited for mass produced microsystems. Relaxation oscillators may be used in a sleep mode instead of a resonator-based oscillator. However, non-resonator based oscillators, such as relaxation oscillators, may exhibit jitter that is higher than the jitter exhibited by resonator-based oscillators for the same level of power consumption. Jitter is the time variation of the clock signal period versus time. Jitter may be caused by various noise sources.
The jitter of a circuit may be caused by jitter due to thermal noise of the circuit and jitter due to flicker noise of the circuit. Various solutions have been proposed to reduce jitter due to thermal noise in a relaxation oscillator, which has been found to be inversely proportional to the reference voltage of the relaxation oscillator. However, such proposed circuits do not address jitter due to flicker noise.
Jitter due to flicker noise is commonly reduced by increasing the size of the transistors in the circuit. However, increasing the size of the transistors in the circuit may not be feasible for mass produced microsystems. Further, the additional capacitance due to the larger transistors may reduce the speed of the circuit and increase the charge injection of the circuit. Benefits may be realized by improvements to relaxation oscillators that reduce the jitter due to flicker noise.